Ferrite single sideband modulator



Jan. 7, 1964 G. B. CRlss FERRITE SINGLE SIDEBAND MoDULAToR 3 Sheets-Sheet 1 Filed Nov. 12, 1958 nella fia/ff: 5. de/ff, y WMM? Jan. 7, 1964 G. B. cRlss 30,117,290y

FERRITE SINGLE SIDEBAND MODULATOR Filed Nov. 12, 1958 3 Sheets-Sheet 2 ym/d@ Jan. 7, 1964 G. B. CRISS FERRITE SINGLE SIDEBAN MODULATOR Filed Nov. 12, 1958 5 Sheets-Sheet 3 @aired Patented dan. 7, i964@ 3,ll7,290 FERRlTE SENGLE SXDEBAND MBULATP. George B. Criss, Moorestown, NJ., assigner to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Filed Nov. l2, i953, Ser. No. 773,Stl7 9 Claims. (Cl. S32-45) The present invention relates to a single sideband modulator and more particularly to a ferrite single sideband modulator for microwave systems.

lIn the past microwave systems such as those employed in various types of radar equipment have employed a separate or local oscillator to supply a signal to mix with another signal of the system `for developing an intermediate frequency signal at the receiver. For accuracy an extremely stable oscillator is required and this results in complex circuitry which includes an automatic frequency control circuit.

`Recently attempts have been made to eliminate the separate oscillator by utilizing various types of known frequency-shift devices operating on a portion of the high frequency energy of the system to provide a suitable mixing signal for developing the necessary intermediate frequency signal. While some success has been achieved such attempts have produced devices having high insertion losses, complexity, and spurious signals.

In brief, the ferrite single sideband modulator of the present invention comprises a microwave transmission system which splits into two arms with a ferrite rotator disposed prior to a short circuit at the terminal end of each arm. The high frequency energy of the system is propagated equally in both arms with a 90 shift in phase in one of the arms. Magnetization of the ferrite is modulated at an intermed-iate frequency by a low power signal to provide a small angle rotation of the plane of polarization of the high frequency energy. This results in a high power mode being reflected back to couple out in an in-phase relationship to subsequent components such as a duplexer and :antenna combination. Also, a lowpower mode, orthogonal to the high-power mode, is developed in each arm by the energy rotation and is separately coupled out of each arm. When the energy of the two orthogonal modes of the arms is added, as by a magic tee, the result is a single sideband signal at a low-power level.

It is, therefore, an object to provide a new and improved single sideband modulator.

Another object is to provide a ferrite single sideband modulator having a negligible insertion loss.

Yet another obiect is to provide Va ferrite single sideband modulator with coupling of the carrier frequency to the output reduced or eliminated by servoing and signal cancellation.

Still another object is to provide a ferrite single sideband modulator which is simple in construction and effrcient in operation.

A further object is to provide a single sideband m-odulator wherein gyromagnetic elements are magnetized in accordance with an intermediate frequency to angle modulate microwave energy of a carrier frequency to develop a single sideband si-gnal at the intermediate frequency.

A still further object lis to provide a single sideband modulator having standard components and very low excitation drive power required for modulation of the magnetization of the ferrite.

Other objects and advantages of the invention will be apparent in the following description and claims considered together with the accompanying drawing in which:

FIG. l is a schematic block diagram of a single sideband modulator as employed in a radar equipment;

FIG. 2 is a perspective vie-w, partly broken away, of a portion of the microwave transmission system of FIG. l, and;

FIG. 3 is a modification of the system of FIG. 2.

Referring to FIG. 1 in detail there is illustrated a single sideband modulator according to the present invent-ion, as employed for example in a radar set. Thus, a source l1 of microwave energy at a carrier or transmitter frequency (fc) suitably energizes a microwave transmission system l2 by means of an input port `13 and the energy is equally divided and propagated in two arms i6 and 17, respectively. One arm 17 `contains a reciprocal phase shifter i8 to provide a 90 shift in the phase of the energy in that arm with respect to the energy in the other arm 16. Disposed in each of the arms lo and 17 is a similar electrically controlled polarization rotator section Z1 and 22, respectively, which will be described in greater detail hereinafter. After rotation, the energy is reflected from shortcircuited ends of the rotator sections 2'1 and 22. and rotated again.

In accordance with the invention an oscillator 23 is provided to develop a low-power signal at an intermediate frequency (fm) and this is applied as a control signal to the rotators 2l and 22 to provide angle modulation of the carrier energy at the intermediate frequency. For proper phasing the output of oscillator 23 to rotator 2.2 is shifted in phase by 90 relative to rotator 2l by a phase shifter 24 suitably connected in the circuit between the oscillator and rotator section 22. Now, with a small angle rotation of the carrier energy in the rotator sections 2l and 22, two components of the energy exist in each arm 116 and 17; namely, a high-power component proportional to the cosine of the angle of rotation and a low power component proportional to the sine of the same angle. The high-power cosine component at the carrier frequency is rellected from the short-circuited ends back toward the input to combine at a centrally disposed and transversely mounted output waveguide 26 `for propagation to an arnplifier 27 4and then through a duplexer 28 to an antenna 2x9. The low-power energy sine component has a sum and difference frequency (fcifm) in the rotator sections 2l and 22 and is individually coupled out by suitable waveguides 31 and 32, respectively, as carrier-suppressed double sideband signals shifted Afrom the carrier frequency by the intermediate frequency.

The two waveguides 33t and 32 are suitably coupled to arms of a magic tee type junction 33 and the double sideband signals add to provide a single sideband signal which is coupled to a mixer 34. A fourth arm of the magic tee 33 is coupled to a matched load in a conventional manner. Now, when a signal of carrier frequency is received by the antenna 29, the received signal is propagated to the duplexer 28 which provides a path for the signal to be applied to the mixer 34A, where the carrier-suppressed single sideband signal acts as local oscillator signal to be combined with the received signal of carrier frequency to eliminate the carrier frequency. Thus, there is available at mixer 34 la signal at intermediate frequency for application to receiver 36, which then converts the information of the received signal to a useable form.

As a means of establishing a reference angle such as Zero angle of rotation of energy in rotater sections 2l and ZZ an adjustable bias is applied thereto such as direct current source 37 with manually-controlled or servocontrolled variable resistors 38 and 39 respectively connected in circuit arrangement therebetween. Thus, stray external effects may be readily compensated to eliminate any errors which may result from such eifects. Control of the variable direct current at resistors 38 and 39 may be utilized to position the electrical center of rotation in each ferrite rotator individually in such a fashion as to cancel and eliminate the direct and quadrature components of carrier frequency which may appear in the single sideband output. In such respect the carrier frequency and the single sideband output are compared by direct and quadrature phase detec-tors which develop errorsignals for altering the values of the bias as respectively applied to the rotator sections `21 and 22.

Referring now to the microwave transmission system 12 in detail, as illustrated in the perspective view of FIG. 2, input port 13'is one port of a magic tee junction and as energy -is applied in the dominant mode atthe carrier frequency, the energy divides equally for propagation in the two arms or branches 16 and 17. None of this incident energy is coupled out waveguide 26`in the conventional rnanner of operation of the magic tee junction; that is, there is magnetic coupling between arms of the magic tee `but no electric coupling. Now, as energy is propagated through the two arms or branches 16 and 17, phase shifter 18, which may be a conventional dielectric plug as shown, introduces a 90 shift in phase in that arm 17 with respect tothe other arm 16 and the reason for such phase shift will be set forth hereinafter.

Each of the arms 16'and 17'is terminated in similar orthogonal mode transducer sections modified as rotators 21 and 22'with ends y41 and 42respectively closed with conducting material to provide short circuits. In addition to the short-circuited ends 41 and 42 the referenced modification includes an element of gyromagnetic material, such as a ferrite rod `43, suitably mounted along the longitudinal axis of the transducer. Now, to magnetize the ferrite rod 43 yalong the direction of energy propagation in a modulated manner and thereby rotate the plane of polarization of the energy, a coil 44`may be wound externally on the transducer with itsraxis 'coincident with the axis of the rod or, as shown in FIG. 2, a coil is wound on the -ferrite rod with leads extended through apertures in a wall of the transducer to an oscillator 23. A second coil 46 is wound in a similar manner with respect to the ferrite rod 43 with leads suitablyextended to the direct current source 37.

In like manner a ferrite rod 51 is disposed in the orthogonal mode transducer and rotator 22 of arm 17 with two coils SZ'and 53 respectively disposed coaxially with respect thereto. The leads of one coil 52 are extended to a 90 phase shifter 294, Iwhich is, in turn, connected to oscillator 23 and the leads of the other coil 53 are extended to direct current source 37.

As stated previously, oscillator 23 generates a low energy signal at an intermediate frequency land this signal is applied to the two ferrite rods `43 and 51 with a 90 phase difference. The value of the signal is established to provide a small angle of rotation of the plane of polarization of the `microwave energy at the carrier frequency in each direction of propagation. By the phrase small angle of rotation is meant an angle on the order of one degree so that the total angle of rotation issubstantially two degrees. Now, with the value of the direct current as respectively applied to coils 46 and 53 established for Zero rotation of microwave energy at the carrier frequency when the oscillator 23 has a zero output, any subsequent rotation due to magnetization of the ferrite rods `43: and 51 in response to the oscillator output will be about a zero reference angle.

The rotation results in a low energy component of the carrier energy equal to the amplitude of the input carrier signal multiplied by the sine of the total angle of rotation which is necessarily a low value because the value of the sine of a small angle, two degrees for example, is an extremely small figure. This low energy component then electromagnetically couples at each transducer-rotator 21 and 22 to the respective transverse waveguide 31 and 312. Since the value is low, the carrier frequency is suppressed and `carries a proportionately large double sideband modulation at the intermediate frequency. The two signals of transverse waveguides 31 and 32 are then combined at the magic tee junction 33 and because of the phase 'shifter 18 in arm l1'7 and phase shifter 24 in the modulation circuit, the two signals are ladded to provide a single sideband signal for application to mixer 34.

in addition to the low energy sine component of the carrier energy, the rotation results in a high energy component equal to the amplitude of the input carrier signal multiplied by the cosine of the total angle of rotation, which in turn is necessarily high because value of the cosine of a small angle is a figure close to one. None of this high cosine component is coupled to waveguides 31 and 32 and so propagates back to the magic tee input. Because of `the phase shifter 18 in arm 17 the energies of ie two arms ld'and 17fadd to couple out waveguide 26 of the magic tee and the sideband modulation at the intermediate frequency is suppressed.

Now, consider'the operation of the device as described in the foregoing'and as utilized, for example, in a conventional radar set. The microwave energy source 11 of transmitter energy (to be amplified later to high power) at a carrier frequency is energized to apply a carrier signal at input port 13 of microwave transmission system 12. The energy divides substantially equally at the magic tee and is propagated along the two arms or branches 16 and 17. The carrier energy of one arm 16 proceeds to transducer-rotator section 21 where the energy is rotated by a small angle in response to the angle modulation at an intermediate frequency applied by the conrbination of oscillator 23, coil 44, and ferrite rod 43. The rotated energy is then reflected by the short-circuited end 41 and rotated in the same direction a second time at the ferrite rod 43. y

The carrier energy of the other arm 17 is shifted in phase by with respect to the energy of arm 16 because of the dielectric phase shifting plug 18. This shifted carrier energy is then angle modulated at the intermediate frequency, in the same manner as set forth for arm 16, by the combination of oscillator 23, coil 52, and ferrite rod 51. It is to be noted that the signal of intermediate frequency is shifted by 90 Vby phase shifter 24 prior to application to coil S2 so that the angle modulation in both arms 16 and 17 has the same relationship with respect to the energy of carrier frequency in the respective arms 16 and 17.

Now, the energy of both arms 16 and 17 has been angle modulated in the same manner and reiiected back toward the input. Because of the angular rotation of the energy, sine and cosine components are established in each arm 16 and 17 respectively. The sine component has a low energy value and has double sideband modulation at the intermediate frequency with suppression of the carrier frequency as it is electromagnetically coupled out by waveguides 31 and 32, respectively. The cosine component has a high energy value at the carrier frequency with little sideband modulation. This cosine component does not couple to waveguides 31 and 32 and is propagated back to the input magic tee where because of the 90 phase shifter 18, the energy adds and is electromagnetically coupled out transverse waveguide arm 26 of the magic tee.

The two low energy double sideband signals of waveguides 31 and 32 are added at magic tee junction 33 to provide a single sideband signal modulated at the intermediate frequency with the carrier frequency suppressed, which is then impressed upon mixer 34. At the same time the high energy component coupled out waveguide 26is applied to amplifier 27 and from the amplifier to conventional duplexer 2S for further application to the antenna 29 without damage to mixer 34. A signal received by antenna 29 in response to the energy of carrier frequency is channeled by duplexer 2S to the mixer 34 Where the received signal is mixed with the previously-described single sideband signal from the magic tee junction 33. The resultant signal at the intermediate frequency as developed by the mixer 34 is impressed upon the receiver 36 for conversion to a desired useable form.

A second microwave transmission system is shown in the perspective illustration of FIG. 3 wherein like reference numerals are utilized for like elements of FIGS. l and 2 as described in the foregoing. Thus, with the structure of FIG. 3 microwave energy of a carrier frequency is introduced at one port 61 of a conventional short-slot hybrid junction 62 which comprises two parallel sections of rectangular waveguide having a common narrow wall 63 with a coupling aperture 64 and impedance matching buttons on the interior surfaces of the two broad walls, one of the buttons being indicated at 67, the other button not being shown in the drawing. The microwave energy is equally divided at the coupling aperture 64 with that portion coupled through the aperture having a 90 degree phase shift with respect to the straight-through portion. The divided energy is then propagated through two sections of rectangular waveguide 68 and e9 to two transducer-rotator sections 21 and 22, which are similar in all respects to those described in detail in the description of the structure of FIG. 2. While the two sections of waveguide 68 and 69 have een shown as having 90 degree bends, such waveguides could readily be two straight sections disposed parallel to one another. It is to be noted that the 90 degree phase shift provided by the aperture 64 of the short-slot hybrid junction 62 eliminates the necessity of introducing a dielectric phase shifter plug 1S as described with respect to the transmission system ll?. of FG. 2.

Consider now the operation of the transmission system of FIG. 3, which operation is substantially the same as described for FIG. 2. Thus, energy of carrier frequency is introduced at port 6l and divided equally at aperture 64 for propagation through waveguide 68 and 69 to transducer-rotator sections 2l and 22. The aperture 64 introduces a 90 degree phase shift to energy coupled through so that the energy of waveguide 69 has a phase difference of 90 degrees with respect to the energy of waveguide 6%. Small angle modulation at an intermediate frequency occurs at the ferrite rods 43 and 51 respectively with the proper phasing between the modulating energy.

The rotated energies are then reflected from the short circuit ends il and 42 and again rotated in the same direction as before. The rotation of the energy results in sine and cosine components with the sine components being electromagnetically coupled out transverse waveguides 3l and 32 to be combined in double sideband carrier-suppressed form and impressed upon mixer 34. At the same time the cosine components propagate back to the short-slot hybrid junction 62 and are recombined, with substantially the same value of power as introduced, for propagation out port 71 to be coupled to antenna 29 in the previously described manner. in other respects the operation follows that previously set forth to provide a single sideband modulator.

Thus, there has been described a single sideband modulator utilizing the characteristics of gyromagnetic material to achieve the stated objects and advantages in a simple manner without a local oscillator and the inherent disadvantages attendant therewith and with very little attenuation of the power of the transmitted energy. While the salient features of the invention have been shown and described with respect to particular embodiments, it will be readily apparent that numerous modifications may be made within the spirit and scope of the invention and it is, therefore, not desired to limit the invention to the exact details shown except insofar as they may be defined in the following claims. What is claimed is:

l. A microwave single sideband modulator comprising waveguide structure for propagating energy at a carrier frequency along two paths having a common input port and short-circuited terminations, separate means disposed in said structure along each path to provide small angle modulation at an intermediate frequency by rotation of the plane of polarization `of the energy at carrier frequency before and after reflection from said short-circuited terminations and to develop sine and cosine components, an output waveguide mounted transversely on said structure adjacent said input port for electromagnetically coupling out said cosine components for a combination of the energies at substantially the same level as input energy, separate waveguides transversely mounted on said structure adjacent each of said sepaarte means for electromagnetically coupling to said sine components, and means connected to said separate waveguides for adding said sine components to provide a single sideband carriersuppressed signal shifted from the carrier by said intermediate frequency.

2. A microwave single sideband modulator comprising waveguide structure having two arms extended along a common center line from a common input port for propagating substantially equal values of energy in each arm at a carrier frequency, phase shifting means mounted in one arm to provide a ninety degree phase shift in either direction of energy propagation with respect to energy in the other arm, an orthogonal mode transducer included as a termination of each arm with a straight-through output port short circuited, means included in each transducer to provide small-angle rotation of the plane of poiarization of energy in accordance with an intermediate frequency and develop sine and cosine components, means coupled to transversely disposed ports of each transducer for adding said sine components to provide a single sideband carrier-suppressed signal shifted from the carrier frequency by the intermediate frequency, and means included in said waveguide structure adjacent said common input port for combining said cosine components to provide an output signal at said carrier frequency having substantially the same power as energy incident at said common input port.

3. The combination of claim 2 wherein said waveguide structure is a magic tee junction.

4. The combination of claim 3 wherein said means for adding said sine componen-ts is also a magic tee junction.

5. The combination of claim 2 wherein said means included in each transducer to provide rotation of the plane of polarization of energy is an element of gyromagnetic material the magnetization of which is varied in accordance with a modulating signal at an intermediate frequency.

6. The combination of claim 5 wherein said element of gyromagnetic material is a ferrite rod disposed with its longitudinal axis lying along a continuation of said cornmon center line and having a coil wound coaxially thereon with leads extended to a source of modulating signal of intermediate frequency.

7. The combination of claim 6 wherein said ferrite rod has a second coil similarly wound thereon with leads extended to a variable source of direct current to provide a biasing magnetic iield for elimination of carrier feedthru.

8. A microwave single sideband modulator comprising a short-slot hybrid junction having two parallel rectangular waveguide sections with a common side wall including a coupling aperture whereby microwave energy at a carrier frequency incident at one port is divided for propagation toward two oppositely disposed and adjacent ports with a ninety degree phase difference, two orthogonal mode transducers respectively coupled to said two adjacent ports and having short-circuited terminations, modulation means disposed in each transducer for providing small-angle modulation of carrier energy at an intermediate frequency to produce sine and cosine components, adjustable bias means coupled to each transducer for rotation of the center of polarization of rotation for elimination of direct and quadrature components of unwanted carrier feed-thru, said cosine components being reflected back to said junction for combination at said junction as an output at carrier frequency at an output port adjacent said one port, and means coupled to transverse arms of said transducers for adding said sine components to provide a single sideband carriensuppressed signal shifted from the carrier frequency by the intermediate frequency.

9. The combination of claim 8 wherein said modulation means disposed in each transducer comprises an element of gyromagnetic material having a coil wound axially thereon providing a magnetizing force varying in accordance with an intermediate frequency.

References Cited in the le of this patent UNITED STATES PATENTS 2,441,598 Robertson May 18, 1948 2,623,993 Altar 1 Dec. 30, 1952 2,820,200 Du Pre 'Jan 14, 1958 2,847,647 Zaleski Aug. 12, 1958 2,872,647 Smith Feb. 3, 1959 2,887,664 Hogan May 19, 1959 OTHER REFERENCES IRE Proceedings, Note on Sidebands Produced by Ferrite Modulators, April 1956, page 556. 

1. A MICROWAVE SINGLE SIDEBAND MODULATOR COMPRISING WAVEGUIDE STRUCTURE FOR PROPAGATING ENERGY AT A CARRIER FREQUENCY ALONG TWO PATHS HAVING A COMMON INPUT PORT AND SHORT-CIRCUITED TERMINATIONS, SEPARATE MEANS DISPOSED IN SAID STRUCTURE ALONG EACH PATH TO PROVIDE SMALL ANGLE MODULATION AT AN INTERMEDIATE FREQUENCY BY ROTATION OF THE PLANE OF POLARIZATION OF THE ENERGY AT CARRIER FREQUENCY BEFORE AND AFTER REFLECTION FROM SAID SHORT-CIRCUITED TERMINATIONS AND TO DEVELOP SINE AND COSINE COMPONENTS, AN OUTPUT WAVEGUIDE MOUNTED TRANSVERSELY ON SAID STRUCTURE ADJACENT SAID INPUT PORT FOR ELECTROMAGNETICALLY COUPLING OUT SAID COSINE COMPONENTS FOR A COMBINATION OF THE ENERGIES AT SUBSTANTIALLY THE SAME LEVEL AS INPUT ENERGY, SEPARATE WAVEGUIDES TRANSVERSELY MOUNTED ON SAID STRUCTURE ADJACENT EACH OF SAID SEPARATE MEANS FOR ELECTROMAGNETICALLY COUPLING TO SAID SINE COMPONENTS, AND MEANS CONNECTED TO SAID SEPARATE WAVEGUIDES FOR ADDING SAID SINE COMPONENTS TO PROVIDE A SINGLE SIDEBAND CARRIERSUPPRESSED SIGNAL SHIFTED FROM THE CARRIER BY SAID INTERMEDIATE FREQUENCY. 